I didn't use one. I put dd underneath in the jump seat and then just unsnapped the top two latches on the seat and stuck ds in his car seat on top. The bar held the car seat in tightly. However, my dd is a tiny tot so it did not make the stroller unstable with the heavier child on the bottom. DS+carseat was only a few pounds lighter than my daughter on the bottom. I have heard that the carseat can fit underneath in the bag area, but I couldn't figure out how to make the chicco seat work there.
Re: Do I need an infant adapter to stick Chico carseat in P&T Sport double?